First of all , I used to prepare the inventorywould make a list what I was going to order .

"I used to" means that in the past you were regularly doing something. "I used to go to that supermarket" "I used to go swimming every week" I'm not sure if this is the timeline you are going for. Are you trying to just describe the procedure in neutral terms, or are you speaking specifically about your experience ordering food?

I will correct assuming that you are talking about your regular ordering procedure in the past. If you still order food like that, you need to change the tenses. For example, this sentence would be "First of all, I make a list..." If you want to talk in general about how someone might order food, "First of all, make a list..." or "First of all, you should make a list..." is correct.

"Inventory" is more like a list of items you have in your company's warehouse or in stock for sale. An inventory contains things you already have.

Than ,Ien, I would visited the onine store's website , meanwhile , enteredenter the correct username and password , clicked the submit button , and, if everything went well , the commodities willitems for sale would appear on the screen .

Websites are online, so you don't have to say "online" too. "Meanwhile" is used for two different things happening at the same time.

"Still ordering this way" version:
Then, I visit the store's website <...> if everything goes well, the items for sale appear on the screen.

Neutral tutorial version:
Then, visit the store's website <...> if everything goes well, the items for sale will appear on the screen.

All food is fresh , I pickedI would then the food I need directly into the trolley in terms of the inventory I prepared before, settleed account after picking,based on the list I had prepared before, pay at checkout, and waited for the exprtransaction to be process ed.

"All food is fresh" is a bit off-topic here. I think it's better to just cut it out, but I tried to squeeze it in.

All the food for sale is fresh, so I would just add the food I need into the trolley based on the list I had prepared before, pay at checkout, and wait for the transaction to be processed.

"Still ordering this way" version:
All the food for sale is fresh, so I just add the food I need into the trolley based on the list I had prepared before, pay at checkout, and wait for the transaction to be processed.

Neutral tutorial version:
All the food for sale is fresh, so I just add the food you need into the trolley based on the list you had prepared before, pay at checkout, and wait for the transaction to be processed.

"Trolley" is British English. The American name for it is a shopping cart.

The whole preocess is easy and fast.